As regards rotating electrical machines such as an axial gap motor, there are known various structures and related technologies.
For example, in a rotor structure for an axial gap motor, which is disclosed in JP2006-166634A, mutually opposite polarity permanent magnets as rotor magnetic poles are alternately disposed in a circumferential direction of a rotor core made of a magnetic material; and the circumferential length of each permanent magnet varies along an axial direction of the permanent magnet so as to increase from an axial gap side (a side facing to a stator) of a rotor to the opposite side thereby so as to form a taper in the circumferential surface of the permanent magnet in order to ensure that a magnetic flux of the permanent magnet on the rotor side concentrate on the side facing an axial air gap of the rotor (on the side facing the stator). Such a rotor core is formed by laminating stamped electromagnetic steel sheets having slots (permanent magnet insertion holes) or by using a powder magnetic core (obtained by mixing iron powder or other magnetic powder with resin or other insulator and solidifying the mixture) with holes.
JP 2007-89270A discloses an axial gap motor in which a rotor is provided with beam-like reinforcements in a radial direction of the rotor and a ring (hoop-shaped member) for retaining the beam-like reinforcement members to increase the rigidity of the rotor. The beam-like reinforcements are joined to the ring through a nonconductive material in order to reduce a loss of an eddy current flowing between the beam-like reinforcement members and the tubular member. A magnetic permeable portion of the rotor is configured with laminated electromagnetic steel sheets or a powder magnetic core which are materials hard to flow the eddy current.
In an axial gap rotating electrical machine, a positional relationship between rotor side magnets (permanent magnets) and a stator core, which are opposed to each other, varies with the rotation of the rotor, and such a variation of the positional relation ship causes a variation of a magnetic flux passing through a rotor. By such a variation of the magnetic flux, the rotor has an occurrence of the eddy current loss.
In order to reduce the eddy current loss, preferably, the material with high resistance is used to reduce the eddy current loss. Laminated electromagnetic steel sheets and a powder magnetic core are generally used as a permeable material to reduce eddy current. However, provided that the electromagnetic steel sheets are laminated in a radial direction of the rotor, a centrifugal force of the rotor becomes great. Therefore, provided that the laminated electromagnetic steel sheets are used as a rotor yoke, the rotor needs a metal reinforcement, a stamping work and a thermal treatment.
Further, since a laminated core comprised of stamped electromagnetic steel sheets has a large residual stress, it needs to use a retention means other than an adhesive. In addition, joining between the permanent magnets and the rotor yoke also needs consideration of enhancing a joint strength therebetween.
As described in JP2006-166634A, in the case of the axial gap motor inserting stepped fan-shaped permanent magnets into holes (slots) formed in the laminated electromagnetic steel sheets, shapes of the rotor yoke and magnets are complicated. As a result, increasing the number of manufacturing process steps, raising the cost of manufacturing, and limiting the degree of freedom in changing the shapes of the magnets.
As described above, in JP2007-89270A, in order to maintain the strength of the laminated electromagnetic steel sheets as a rotor yoke, the axial gap motor is provided with plural beam-like reinforcements between each pair of magnets and a hoop-shaped member attached to the outer circumference of a rotor core. A nonmagnetic material since is provided between the beam-like reinforcement members and the hoop-shaped member, no eddy current flows to a loop of the reinforcements. However, the hoop-shaped member since is a ling in form, if no particular provision is made, it is highly probable that the hoop-shaped member may allow an eddy current to flow to the outer circumference. It should also be noted that the use of the reinforcements is likely to make an assembly process complex and increase the cost.
The present invention is proposed in view of the above circumstances, and provides an axial gap rotating electrical machine capable of reducing the eddy current loss in the rotor yoke, maintaining the strength of the rotor without using a complex structure, making the assembly process simple and easy, and reducing the cost. The present invention also provides a rotor for use in axial gap rotating electrical machine.